ARM: 8289/1: dma-mapping: use to_dma_iommu_mapping instead of accessing archdata

When using the IOMMU-backed DMA ops for a device, we store a pointer to
the dma_iommu_mapping structure (used to keep track of the address
space) in the archdata.mapping field of the struct device.

Rather than access this field directly, use the to_dma_iommu_mapping
helper in dma-mapping, so that we don't really care where the mapping
information is held.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>